About 40 years ago, researchers first began to suspect that we have neurons in our brains called “place cells.” They’re responsible for helping us (rats and humans alike) find our way in the world, navigating the environment with some internal sense of where we are, how far we’ve come, and how to find our way back home. All of this sounds like the work of maps. But our brains do impressively sophisticated mapping work, too, and in ways we never actively notice.
Every time you walk out your front door and past the mailbox, for instance, a neuron in your hippocampus fires as you move through that exact location – next to the mailbox – with a real-world precision down to as little as 30 centimeters. When you come home from work and pass the same spot at night, the neuron fires again, just as it will the next morning. “Each neuron cares for one place,” says Mayank Mehta, a neurophysicist at UCLA. “And it doesn’t care for any other place in the world.”
This is why these neurons are called “place cells.” And, in constantly shuffling patterns, they generate our cognitive maps of the world. Exactly how they do this, though, has remained a bit of an enigma. The latest research from Mehta and his colleagues, published this month in the online edition of the journal Science, provides more clues. It now appears as if all of the sensory cues around us – the smell of a pizzeria, the feel of a sidewalk, the sound of a passing bus – are much more integral to how our brains map our movement through space than scientists previously believed.
And the more scientists learn about how our brains construct cognitive maps of space, the more we may learn about how to design those spaces – streets, neighborhoods, cities – in the first place. Or, rather, we may learn more about the consequences of how we’ve built them so far. How could any urban planner, for starters, not love the idea that “place” is embedded in the brain?
When I called Mehta in California to ask him to explain this to me, he started, quite patiently, from the very beginning.
“All animals must move – that’s why they are called ‘animals,’ they’re animates,” he says. “In fact, that’s what they evolved to do compared to plants.”
But movement through space costs energy. And evolutionarily speaking, we’d all be in terrible shape if we never knew where we were, or if we spent days wasting energy and wandering back to the nest every time we left it. As a result, animals (including human beings) have evolved to move through space as efficiently and effortlessly as possible. Finding our way is, in short, one of the most fundamental things we do.
“The question is how, then?” Mehta says. “How is it that our minds and our bodies together, going somewhere, are able to quickly perceive this abstract thing called space?”
Until now, scientists believed our cognitive maps were primarily built using two kinds of cues: external visual landmarks (the 7-Eleven across the street, the mountains on the horizon), and our internal sense of motion (how fast we move, generating an awareness of distance). But of course other kinds of sensory stimuli can also connect us to place (or confuse us about where we are). If you smell something burning in your kitchen, for example, that’s probably not too alarming. “But if you smell a burning smell in the bedroom,” Mehta says, “that’s a disaster.” Running water in the bathroom? No problem. Running water in the living room? Big problem. In the same way, we don’t expect to feel gravel underfoot inside a sandwich shop, or to smell Kung Pao Chicken coming from a storefront that’s long been a pizzeria.
“Many of these things are seemingly unconscious,” Mehta says. “But we are using those things so that, broadly speaking, there are many, many stimuli we’re using to generate maps of space.”
Mehta and his colleagues have proven this researching – of course – rats. Even for rats, it’s impossible to truly eliminate all sensory cues in the real world (crumbs of food, scent marks, spilled droplets of liquid). And so they created a virtual reality environment. The rats were placed atop a tracking ball, affixed to the apparatus in a little vest (a scientific aside: “Within a day or two,” Mehta says, “they like the idea of having a vest around them. They literally walk around with the vest, which is pretty funny.”). The rats were then immersed within a virtual-reality screen picturing a room with walls decorated in different blue and green shapes. They then ran around said virtual room atop the treadmill ball.
A) The virtual reality system on a spherical treadmill. B) A top-down view of the environment. C) A rat in the actual apparatus. From “Multisensory Control of Hippocampal Spatiotemporal Selectivity” supplemental materials in Science.
Later, the researchers repeated the same experiment within a visually identical actual room, 10 feet by 10 feet in size. The results? About 60 percent of the place cells that fired in the rats’ brains inside the actual room, with all its sensory stimuli, were inactive in the virtual space. Which means all of that other stuff – the equivalent, for humans, of a subway grate underfoot, or the smell of Lake Michigan to your East – seems to matter a great deal.
In the real world, when you walk the same route every day to the bus stop, your brain begins to use all of those cues to anticipate where to go next. “When you are walking, if you really think about it for a minute, just knowing where you are is the tiniest part of the story,” Mehta says. “The biggest part is ‘what should you do next?’ Should you stay put, walk forward, go back, go left?”
Your brain actually goes from living in the present to anticipating the future (try that, Google Maps!). “We believe this amazingly complex set of things – environmental landmarks, our self-motion, brain rhythms, smells and textures – all of that is coming together to tell us what we should do next in space,” Mehta says.
All of this raises a fascinating question for the professions more involved in creating places and neighborhoods than studying how people move through them. If all the stimuli from our environment is directly connected to how our brains navigate through space, does that mean that some types of environments enable our brains to build better maps? What if you’re walking through a neighborhood where all the visual cues – the houses – look the same, or where there’s not much to smell at all? The right answer, Mehta says, is not so simple.
“We have to think a little more carefully about what is the right kind of environment that we should be building that’s better for communities to live in,” Mehta says. “What is the right level of cognitive challenge that must be in the environment? We don’t want to run a marathon each day.”
Of course, scientists will have to learn a lot more about how humans process actual environments – and not just how rats scurry over a virtual treadmill – to be able to answer this question. But imagine urban planners one day drafting plans for a street or a neighborhood using insights from the maps that we make in our minds.
“The fact is this is an emerging science,” Mehta says. “But this is a great conversation to have.”
